1. Field of the Invention
This invention relates to a photochemical vapor deposition apparatus.
2. Description of the Prior Art
Recently, there, have been studied methods for forming a vapor-deposited film of amorphous silicon for use in the photosensitive drum of a duplicating machine or a solar cell. On the other hand, a vapor depositing method is further utilized in the formation of diverse insulating films or protective films, and a variety of vapor depositing methods have been proposed in answer to various uses. Among these methods, a photochemical vapor depositing method utilizing a photochemical reaction is being particularly watched now because it has such advantages as the film-forming rate being remarkably high and a uniform film being formable on a portion of large area of a substrate, too.
A conventional chemical vapor depositing method utilizing a photochemical reaction comprises placing a substrate in an air-tight vessel made of material through which ultraviolet rays can be fully transmitted, feeding a photoreactive gas to flow through the vessel and applying ultraviolet rays radiated from an ultraviolet discharge lamp outside of the vessel through the wall thereof onto the substrate so that a photochemical reaction is caused to decompose the photoreactive gas and the resulting reaction product is vapor-deposited onto the substrate. In spite of having the above-mentioned remarkable advantages, this conventional photochemical deposition method, that may be called an "outer discharge type", has been found to have the defect that the reaction product is also vapor-deposited on the inner wall of the vessel, thereby seriously impeding the transmission of ultraviolet rays.
Thus, a photochemical vapor deposition apparatus, that may be called an "inner discharge type", has been studied and developed. In the apparatus of this type, a reaction space and a discharge space are surrounded by the same in an airtight manner. The reaction space forms a passage for a photoreactive gas and in this reaction space a substrate is placed. In the discharge space, an electric plasma discharge is generated and ultraviolet rays radiated from the plasma are directed onto the substrate to cause a photochemical reaction of decomposition of the photoreactive gas. Between the plasma and the substrate, there is no partition member that impedes passing of the ultraviolet rays.
In the above-mentioned apparatus, a part of the plasma of electric discharge between electrodes diffuses or expands and the vapor-deposited film which is being formed on the substrate is damaged by the plasma. In order to prevent such damaging of the film, it is required that the substrate be placed at a position more remote than the mean free path of ions or electrons produced in the plasma. To carry out vapor deposition with high efficiency, however, it is necessary to keep the substrate near to the ultraviolet ray source, i.e. the plasma, as much as possible, so that ultraviolet rays of larger intensity reach the substrate.